The present invention relates to a self-locking nozzle block for use in axial flow steam turbines.
Axial flow steam turbines contain a rotor that is situated in a casing, or a pair of spaced casings, an outer casing and an inner casing that contains the rotor. In the inner casing, nozzle chambers are provided which change the direction of inlet steam from a radial to an axial direction and then through nozzle blocks that direct the steam to the blades and vanes of the turbine.
In operation of axial flow steam turbines, incoming steam is charged through inlet nozzles to a nozzle ring that contains a plurality of nozzle blocks, the nozzle blocks containing vanes that direct the steam to the control stage or first stage of expansion of the steam. It is important that the nozzle blocks be securely situated in the nozzle ring and vibration of the blocks prevented. The use of threaded fasterners to secure the nozzle block in the nozzle ring can cause problems when such threaded fasteners loosen and/or fail under high vibration or high temperatures. Such threaded fasteners are subject to severe stresses and failure is intensified because their rate of expansion is different than those of the nozzle blocks and ring. Failed fasteners can cause problems, such as loose blocks and other foreign fragments. Vibrations could cause the loose nozzle blocks to damage the rotors while fragments of broken fasteners could damage any adjacent materials and migrate to cause damage elsewhere.
Other locking members, such as locking keys have been prepared to secure nozzle blocks or shrouds in place. In U.S. Pat. No. 3,021,110, for example, a key is affixed to a turbine shell having a thermal coefficient of expansion substantially different from that of the turbine shell, the key affixed to the shell by a locating screw. The key's greater expansion at high temperatures closes a clearance gap between the key and the shell cooperating surfaces to secure a turbine nozzle rim and hold it in place. In that system, the expanding key needs a threaded fastener to hold it in place. This threaded fastener is subject to similar stresses and breakage potential as threaded fasteners used to directly affix a nozzle block in a nozzle ring. Additional damage could occur if the key itself were loosened or broken. The expanding key in that system fits between the nozzle ring or chamber surface and the nozzle block surface and wedge them together when expanded, and is proposed for use on the outer shroud only, with conventional bolts used to affix the nozzle blocks to the inner shroud, which bolts are subject to the stresses and breakage potential described previously. Also, when using such an expanding key, it is required that the nozzle ring be shaped with a considerable overhang which could cause excessive vibration.
It is an object of the present invention to provide a method for locking a nozzle block in a nozzle ring at high temperatures in a manner that does not require the use of threaded fasteners, welding, or other mechanical locking devices.